1. Field of the Invention
The present invention relates to an optical head and optical disk apparatus for recording to or reproducing from a rotating disk-form information medium (hereinafter called the “optical disk”), and in particular, relates to an optical head and optical disk apparatus which carry out focus pull in action without causing collisions between optical disk and condensing lens for collecting optical beams when recording or reproduction (playback) is carried out.
2. Description of Related Art
In a conventional optical disk apparatus, when signals are reproduced, the optical disk is irradiated with an optical beam of comparatively weak specified light amount and the reflected light modulated strongly or weakly by the optical disk is detected. In addition, when signals are recorded, the light amount of optical beams is modulated strongly or weakly in accordance with signals recorded and information is recorded in a recording material film on the optical disk. This is described in, for example, Japanese Laid-Open Patent Publication No. 52-80802, etc.
The read only optical disk records the information on pits recorded in advance in the form of a spiral. In addition, the optical disk which can be recorded and can be reproduced has optically recordable and reproducible material film formed on the substrate surface with spiral-form track of patterned indented structure by techniques such as evaporation. In order to record information on the optical disk or to reproduce information recorded, focus control is required to control optical beam in the direction normal to the optical disk surface (hereinafter called the “focus direction”) so that optical beams achieve constantly the specified convergence condition on the recording material film.
Referring now to FIG. 17A, a control of conventional optical disk will be described. Optical head 10 is provided with semiconductor laser 11, coupling lens 12, polarizing beam splitter 13, ¼ wavelength plate 14, condensing lens 15, focus actuator which is a focus moving means (hereinafter called the “Fc actuator”) 16, tracking actuator (hereinafter called the “Tk actuator”), detection lens 18, cylindrical lens 19, and optical detector 20.
The optical beam generated from the semiconductor laser 11 is made into parallel beam by the coupling lens 12, passes the polarizing beam splitter 13 and the ¼ wavelength plate 14, and is converged onto the optical disk 1 by condensing lens. The optical beam reflected against it again passes the condensing lens 15 and ¼ wavelength plate 14, is reflected at the polarizing beam splitter 13, passes the detection lens 18 and the cylindrical lens 19, and is applied to the optical detector 20 with a light receiving section divided into four. The condensing lens 15 is supported by elastic material such as springs, When the Fc actuator 16 is energized, the condensing lens 15 moves to a direction substantially perpendicular (focus direction) to the optical disk 1 by electromagnetic force. The optical detector 20 which has a light receiving section divided into four sends each light amount signal detected to the focus error generator 30 (hereinafter called the “FE generator 30”).
By using each light amount single from a light receiving section divided into tour, the FE generator 30 obtains error signal (hereinafter called the “FE signal”) that indicates the converging condition of the optical beam on the information plane of the optical disk 1 and sends it to the Fc actuator 16 via the focus control filter 31 (hereinafter called the “Fc filter”), the drive signal selector 32, and the focus drive 33 (hereinafter called the “Fc driver”). In addition, the FE generator 30 sends the FE signal to a pull in signal generator 71.
FIG. 17B is a diagram showing a light receiving section of the optical detector 20. As shown in the figure, the optical detector has four light receiving sections 201a through 201d. Because the astigmatism is generated due to a cylindrical lens, if the condensing lens 15 moves in the direction that approaches the optical disk 1, the focus of the optical beam on the optical detector 20 becomes ellipse elongated in directions of a light receiving section 201a and a light receiving section 201c. In the event that the condensing lens 15 moves in the direction separating from the optical disk 1 with respect to the focalized point, the focus of the optical beam on the optical detector 20 becomes ellipse elongated in directions of a light receiving section 201b and a light receiving section 201d. FE generator 30 generates the FE signal by calculating the difference between the light amount sum in light receiving section 201a and light receiving section 201c and light amount sum in light receiving section 201b and light receiving section 201d. This technique is called “an astigmatism method”. The Fc actuator 16 drives the condensing lens 15 in the focus direction so that the optical beam converges on information plane of the optical disk 1 in the specified condition. Focus control is done as described above.
Search-driving generator 64 which is a means for generating search driving operation generates a drive signal for driving the Fc actuator 16 in the approaching direction or separating direction at a specified speed. The search-driving generator 64 sends a signal to the Fc actuator 16 via the drive signal selector 32 and the Fc driver 33. The signal from the optical detector 20 are sent to the reflected light amount detector 70. The reflected light amount detector 70 operates sum signals from all the light receiving sections of the optical detector 20 and sends the summed signal to the pull in signal generator 71. The pull in signal generator 71 sends a low-level signal to the drive signal selector 32 when the signal from the reflected light amount detector 70 is greater than the specified level L1 and until the FE signal from the FE generator 30 passes the reference level from positive to negative. Thereafter, pull in signal generator 71 sends high-level signals to drive signal selector 32. Drive selector 32 sends a signal from the Fc filter 31 which is a focus control driving means to FC driver 33 when the signal from pull in signal generator 71 is of a high level, or sends the signal from the search-driving generator 64 to the Fc driver 33 when the signal from the pull in signal generator 71 is of a low level. The Fc driver 33 drives the Fc actuator 16 based on the signal selected by the drive signal selector 32.
Referring now to FIGS. 18A and 18B, description will be made on FE signal from the FE generator 30 and signals from the reflected light amount detector 70, which are detection signals. FIG. 18A is a diagram showing one example of FE signal output from the FE generator 30 and FIG. 18B is a diagram that shows one example of output signal from the reflected light amount detector 70. The abscissa of FIG. 18 is the relative position in the focus direction of the optical disk 1 and the condensing lens 15 and is an approaching direction from left to right in the figure. In addition, the ordinates of FIG. 18A and 18B show the respective signal level. As shown in FIG. 18A, the FE signal from the FE generator 30 is positive in the separating direction from the focalized point and negative in the approaching direction. As shown in FIG. 18B, the signal from the reflected light amount detector 70 becomes cone-shaped signals in the detection range nearly same as that of the FE signal.
Referring now to FIGS. 19A through 19D, the focus pull in operation is described. FIG. 19A is a diagram showing one example FE signal output from the FE generator 30, FIG. 19B is a diagram showing one example of an output signal from the reflected light amount detector 70, FIG. 19C is a diagram showing one example of an output signal from the drive signal selector 32, and FIG. 19D is a diagram showing one example of an output signal from the pull in signal generator 71. The abscissa of each of FIG. 19A through FIG. 19D shows a time, and the ordinate shows a level of the respective signal. The polarity of the signal from the drive signal selector 32 required for condensing lens 15 to move in the approaching direction to optical disk 1 is positive in FIG. 19C. As shown in FIG. 19D, because the signal from pull in signal generator 71 is of a low level at the start of focus pull in, the drive signal selector 32 sends the signal from the search-driving generator 64 to the Fc driver 33. As shown in FIG. 19C, the search-driving generator 64 generates driving operation so that the condensing lens 15 moves in the direction for approaching the optical disk 1 from the condition in which the optical disk 1 and the condensing lens 15 are sufficiently separated. When the focus of optical beams approaches a focalized point on the information plane of the optical disk 1, as shown in FIG. 19A and FIG. 19B, a change occurs in the FE signal from the FE generator 30 and a signal from the reflected light amount detector 70.
The pull in signal generator 71 makes a signal to drive signal selector 32 to a high level when the signal level from the reflected light amount detector 70 exceeds L1 and the FE signal from the FE generator 30 crosses the reference level from positive to negative. Thereafter, the signal from the Fc filter 31 is selected by the drive signal selector 32 and focus control is brought into an active condition (an operating condition).
In recordable optical disk 1, recording action is carried out by absorbing optical beam with which the information plane of the optical disk is irradiated, that is, by changing the phase of recording material by heat accumulated by the absorption. Consequently, the absorption ratio of information plane of the optical disk 1 must be increased to improve a recording speed, and in such event, the reflectivity tends to lower. Furthermore, since in the optical disk which has a plurality of information planes for improved recording capacity, the information planes on the far side as seen from condensing lens 15 must be irradiated with optical beams, transmittance on the information plane on the near side as seen from condensing lens must be increased. That is, the reflectivity of the information plane of the optical disk tends to lower as the recording capacity and recording speed are improved. In addition, the reflected signal of the optical beam occurs not only from the information plane of the optical disk but also from the surface of the optical disk. Because when the reflectivity in the information plane of the optical disk 1 lowers to the level equivalent to the reflectivity at the surface of the optical disk, the FE signal from the FE generator 30 and the signal from the reflected light amount detector 70 begin to have same waveforms on the surface and the information plane of the optical disk 1, they are unable to be distinguished during focus pull in operation, and the focus control is carried out to the surface of optical disk 1. As a result, a problem of long time required for irradiating the desired address with optical beams occurs.
Furthermore, a beam spot of the optical beam must be made smaller in order to improve recording density of the optical disk 1. In general, achieving a small beam spot by shortening the focal length of condensing lens 15 shortens the distance from a focalized point on the information plane to colliding position and increases the possibility of collision between the condensing lens 15 and the optical disk 1. Consequently, a means to solve the problem for the equivalent reflectivity on the information plane and the surface of optical disk is proposed by carrying out focus pull in operation while the beam spot of the optical beam is kept away from the optical disk 1 after the condensing lens 15 is moved in such a manner that the beam spot of the optical beam is located on the optical disk 1 side rather than on the information plane of the optical disk. However, in the case that the distance from the focalized point on the information plane to the colliding position is smaller than the face-runout of the optical disk 1, it is impossible to bring the condensing lens 15 to the optical disk 1 from the information plane focalized point without collision and thus there is a big problem of inevitably damaging optical disk 1.